Human Reproduction Update, Vol.8, No.4 pp. 297±311, 2002
The molecular basis of sperm±oocyte membrane
interactions during mammalian fertilization
Janice P.Evans
Department of Biochemistry and Molecular Biology, Division of Reproductive Biology, Bloomberg School of Public Health,
615 N. Wolfe St, Johns Hopkins University, Baltimore, MD 21205, USA. E-mail: [email protected]
Gamete membrane interactions begin with adhesion (binding) of the sperm to the oocyte plasma membrane and
culminate with fusion of the membranes of the gametes, thus creating the zygote through the union of these two very
different cells. This review summarizes the molecular and cell biology of the cell±cell interactions between
mammalian gametes. Recent research studies have provided new insights into the complexity of these interactions
and into the importance of multimeric molecular networks and optimal membrane order in both sperm and oocytes
for successful fertilization. Molecules that will be highlighted include cysteine-rich secretory protein 1 (CRISP1) and
ADAMs [fertilin a (ADAM1), fertilin b (ADAM2) and cyritestin (ADAM3)] on sperm, and integrins, CD9, and other
integrin-associated proteins on oocytes, as well as other molecules. The characteristics of these gamete molecules are
summarized, followed by discussions of the experimental data that provide evidence for their participation in gamete
membrane interactions, and also of the speci®c roles that these molecules might play. Insights from a variety of
research areas, including gamete biology, cell adhesion, and membrane fusion, are put together for a tentative model
of how sperm±oocyte adhesion and fusion occur. The clinical relevance of correct gamete membrane interactions is
also noted.
Key words: ADAM/CD9/cell adhesion/integrin/membrane fusion
TABLE OF CONTENTS
Introduction
Microscopic studies: key insights into sperm±oocyte membrane
interactions
Molecules that mediate gamete membrane interactions
A model of how mammalian sperm±oocyte adhesion and fusion may
occur
Concluding thoughts
Acknowledgements
References
Introduction
Mammalian sperm interact with oocytes on three different levels
during fertilization: (i) the cumulus layer; (ii) the zona pellucida
(ZP), which induces exocytosis of sperm acrosome contents; and
(iii) the oocyte plasma membrane, which begins with adhesion (or
binding) to the oocyte plasma membrane and concludes with
fusion of the sperm membrane with the oocyte membrane. This
review will focus on this third level, the interactions between
gamete membranes. This interaction is preceded by acrosomal
exocytosis, which is triggered by sperm binding to the ZP and is a
critical prerequisite step to sperm±oocyte membrane interactions.
It not only gives the sperm access to the perivitelline space where
Ó European Society of Human Reproduction and Embryology
membrane interactions take place, but also exposes and modi®es
regions of the sperm surface to make the sperm capable of
interacting with the oocyte membrane. Gamete membrane
interactions involve a complex series of molecular interactions,
beginning with initial attachment of the sperm to the oocyte,
leading to ®rm cell±cell adhesion, and culminating in fusion of
the two membranes, making one cell out of two. This review will
address the cellular and molecular bases of these processes.
The main experimental method used to study the cellular and
molecular mechanisms of sperm±oocyte interactions has been the
IVF assay, examining sperm±oocyte interactions in experimentally manipulated conditions. For assessing gamete adhesion and
fusion, ZP-free oocytes are usually used. The design of
experiments with regard to insemination condition variables,
assessment of sperm±oocyte adhesion and sperm±oocyte fusion,
and interpretation of results from such assessments have been
discussed (Evans, 1999). Data from IVF experiments have been
augmented by other work, including studies of cellular interactions (such as with cells transfected to express gamete proteins of
interest) and molecular interactions (such as analyses of speci®c
protein±protein interactions).
Most of what is known about gamete interactions has been
derived from animal models. Many of the molecules involved in
fertilization processes that have been identi®ed in mammalian
model systems are conserved in humans. Studies of heterologous
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systems (sperm and oocytes from different species) can also be
performed. ZP-free hamster oocytes are commonly used for
studies of membrane interactions because they fuse with sperm
from nearly every mammalian species tested (Yanagimachi,
1988). It has generally been assumed that if a reagent (e.g. an
antibody) inhibits the interaction of human sperm with hamster
oocytes, then it would also inhibit the interaction of human sperm
with human oocytes. While this is likely to apply in most cases,
there are examples of `false positives' [i.e. a reagent that inhibits
in the heterologous system but not in the homologous system
(Primakoff and Hyatt, 1986)] and `false negatives' [i.e. a reagent
that inhibits in the homologous system but not in the heterologous
system (Okabe et al., 1988)]. Thus, care is necessary in the
interpretation of results from the ZP-free hamster oocyte assays
until more is known about the molecules involved in gamete
membrane interactions. In addition, the fusibility of the hamster
oocyte plasma membrane could be due to unique features of the
biochemical nature of that particular membrane environment
(lipids and/or proteins). As such factors are not yet completely
understood, the relevance of the ZP-free hamster oocyte
penetration test to bona ®de human sperm±oocyte membrane
interactions remains to be determined.
How have molecules involved in gamete membrane interactions been identi®ed? The `candidate molecule' approach has
been applied, basing studies of sperm±oocyte interactions on
studies of other cell±cell interactions. For example, oocytes and
sperm have been examined for the expression of known cell
adhesion molecules, such as integrins, extracellular matrix
proteins, cadherins and immunoglobulin superfamily cell adhesion molecules. Data exist that support the role of integrins in
sperm±oocyte interactions (see below). In addition, some sperm
molecules were identi®ed by what might be considered proteomic
methods, used well before the word `proteome' was coined. One
example of this is the generation of a collection of anti-sperm
monoclonal antibodies that are likely to cross-react with surface
epitopes by immunizing mice with intact sperm or to sperm
membrane proteins. These antibodies are then screened for the
ability to bind to sperm and to perturb sperm function (such as
sperm±zona binding, sperm±oocyte interaction, etc.). Details of
some of the molecules identi®ed by this and other methods are
provided below.
Microscopic studies: key insights into sperm±oocyte
membrane interactions
Early insights into gamete plasma membrane interactions came
from elegant microscopic analyses, including light microscopy,
scanning and transmission electron microscopy, and video
microscopy (Yanagimachi, 1988, 1994; reviews). The oocyte
plasma membrane is covered with microvilli; in rodent oocytes,
the region overlying the meiotic spindle is free of microvilli, and
sperm±oocyte fusion rarely occurs in this region (Ebenspaecher
and Barros, 1984). Sperm interactions with the oocyte membrane
also occur in a spatially restricted manner, with the inner
acrosomal membraneÐwhich is exposed after the acrosome
reactionÐcontacting the oocyte membrane ®rst (Huang and
Yanagimachi, 1985). Subsequently, the equatorial segment and
the posterior head of the sperm adhere to and then fuse with the
oocyte membrane (Yanagimachi and Noda, 1970; Bedford et al.,
298
1979). In the rodent, acrosome-intact sperm can adhere to the
oocyte membrane, but only acrosome-reacted sperm fuse with the
oocyte membrane (Yanagimachi and Noda, 1970; Phillips and
Yanagimachi, 1982). In human sperm, there is evidence that the
acrosome reaction is important for adhesion to the oocyte
membrane (Talbot and Chacon, 1982; Bronson et al., 1999a).
Initial attachments of the sperm to the oocyte membrane are
reversible and appear to require sperm motility (Wolf and
Armstrong, 1978), although sperm with poor motility can fuse
with oocytes (Yanagimachi, 1988). Sperm tail movement
decreases or stops within a few seconds of sperm±oocyte fusion
(Wolf and Armstrong, 1978). Electron microscopy shows that the
inner acrosomal membrane is later engulfed by the oocyte through
a process that appears similar to phagocytosis (Huang and
Yanagimachi, 1985). The sperm tail is also eventually incorporated into the oocyte (Hirao and Hiraoka, 1987).
Molecules that mediate gamete membrane interactions
Sperm-associated cysteine-rich secretory protein 1 (CRISP1) (DE,
AEG-1 and ARP)
The protein known as DE was identi®ed in rat epididymal protein
lysates in an analysis of proteins expressed in the epididymis in an
androgen-dependent manner (Cameo and Blaquier, 1976). The
protein complex gets its name from two bands, D and E, on a
denaturing polyacrylamide gel. Proteins D and E co-purify, are
antigenically related, and are likely variants of the same gene
product based on the high amino acid identity between tryptic
peptides of protein D and protein E, although there are some
differences between D and E (including size, high-performance
liquid chromatographic separation of tryptic peptides, monoclonal
antibody recognition and epididymal expression patterns)
(Brooks, 1982; Xu and Hamilton, 1996). The molecular basis of
the differences between proteins D and E is not known.
The peptide sequences of proteins D and E in the rat (Xu and
Hamilton, 1996; Cohen et al., 2000b) and the mouse epididymal
protein (MEP)-7 epididymal antigen in the mouse (Rankin et al.,
1992) match the deduced amino acid sequence of previously
characterized cDNA clones, an androgen-regulated sperm-coating
epididymal protein in rat and rat acidic epididymal glycoprotein
(AEG)-1 (Brooks et al., 1986; Charest et al., 1988). With the
identi®cation of additional related proteins in several mammalian
species, this family has come to be known as the CRISP family.
Family members include DE-like proteins expressed in the
epididymis (Haendler et al., 1993; Eberspaecher et al., 1995;
KraÈtzschmar et al., 1996a; Sivashanmugam et al., 1999), another
family member, CRISP2, expressed primarily in the testis (also
known as Tpx-1) (Kasahara et al., 1989; Foster and Gerton, 1996;
KraÈtzschmar et al., 1996a; O'Bryan et al., 1998), and another,
CRISP3, with variable tissue distribution (Mizuki and Kasahara,
1992; Haendler et al., 1993; KraÈtzschmar et al., 1996a;
Schambony et al., 1998a). Cysteine-rich carboxy-terminal portions are present in all CRISPs, and the overall amino acid
identities between CRISP family members range from ~30% to
>80%. Epididymal proteins D and E are believed to be two forms
of rat CRISP1. Mammalian CRISPs are closely related to certain
toxins in reptiles. The vertebrate CRISPs are more distantly
related to pathogenesis-related proteins (PR1) in plants and
Gamete plasma membrane interactions
similar proteins in fungi; venom polypeptides in insects; and a
Xenopus sperm chemoattractant protein that is synthesized in the
oviduct and deposited with oocyte jelly coat (Foster and Gerton,
1996; Hayashi et al., 1996; Olson et al., 2001).
CRISP1/DE has been studied most extensively in the rat and,
unless otherwise noted, the data discussed below refer to that
species. CRISP1 is synthesized by the epididymis and associates
with rat sperm during epididymal transit. Expression of the D and
E forms of CRISP1 protein varies by epididymal region (Brooks,
1982; Moore et al., 1994), although the differences do not seem to
be due to alternative splicing of CRISP1 mRNA (Klemme et al.,
1999; Roberts et al., 2001). Most association of CRISP1 with
sperm is evident in sperm retrieved from the distal corpus and
cauda epididymis (Moore et al., 1994). There appear to be two
`levels' of CRISP1 association with sperm. The majority of
CRISP1 is loosely associated with rat sperm, and is released
during capacitation in vitro (Cohen et al., 2000b) or by
biochemical extraction treatments (Kohane et al., 1980; Rankin
et al., 1992; KraÈtzschmar et al., 1996a). A fraction of CRISP1
may be tightly associated with rat sperm (Wong and Tsang, 1982;
Moore et al., 1994; Cohen et al., 2000b), although the amount of
CRISP1 that remains associated with sperm and/or the strength of
the association may vary by species (Rankin et al., 1992;
KraÈtzschmar et al., 1996a; Cohen et al., 2001). Other CRISP
family proteins expressed in the male reproductive tract may
associate with sperm in some species (Schambony et al., 1998b).
It is not known how rat CRISP1 associates with the sperm
membrane; there are data to indicate that rat sperm-associated
CRISP1 is not attached through a glycosylphosphatidylinositol
(GPI) linkage, directly or indirectly (i.e. to a GPI-linked binding
partner) (Moore et al., 1994). Immuno¯uorescence data show that
sperm-associated CRISP1 is localized on the dorsal region of the
acrosome of cauda epididymal rat and mouse sperm (Rochwerger
and Cuasnicu, 1992; Cohen et al., 2000a); with one antibody, it is
detected on the tail of cauda epididymal and ejaculated rat sperm
(Moore et al., 1994; Xu et al., 1997). On capacitated and
acrosome-reacted rat sperm, CRISP1 is detected on the equatorial
segment (Rochwerger and Cuasnicu, 1992). In ejaculated sperm
from the horse, human and rhesus monkey, CRISP proteins are
detected on the principal piece and mid-piece of the tail as well as
the posterior (post-acrosomal) region of the sperm head (Hayashi
et al., 1996; Schambony et al., 1998b; Sivashanmugam et al.,
1999).
CRISP1 has been implicated in rodent gamete membrane
interactions. Anti-DE antibodies inhibit fertilization of rat
oocytes, either when mixed with rat sperm before the sperm are
used for arti®cial insemination (Cuasnicu et al., 1984) or in IVF
(Cuasnicu et al., 1990). Additionally, CRISP1 protein puri®ed
from rat epididymal extracts binds to the plasma membrane of rat
and mouse oocytes, and rat oocytes and mouse oocytes treated
with puri®ed rat CRISP1 show reduced levels of fertilization
when mixed with sperm in IVF (Rochwerger et al., 1992; Cohen
et al., 2000a). Male rats immunized with CRISP1 show reduced
fertility in mating trials, as well as the presence of anti-CRISP1
antibodies in epididymal and vas deferens ¯uids; sperm from
these immunized males could adhere to but not fuse with ZP-free
oocytes (Ellerman et al., 1998). In agreement with this, puri®ed
CRISP1 was found to inhibit rat sperm±oocyte fusion but was
apparently without effect on sperm±oocyte adhesion (Rochwerger
et al., 1992; Ellerman et al., 1998). A CRISP protein might also
participate in human gamete membrane interactions. A human
epididymal protein with some homology to rat CRISP1 (38%
amino acid identity) has been described as AEG-related protein
(ARP) or the human orthologue of rat CRISP1 (Hayashi et al.,
1996; KraÈtzschmar et al., 1996a); a similar protein has also been
characterized in the rhesus monkey (Sivashanmugam et al.,
1999). There are con¯icting data regarding how much ARP is
tightly associated with human sperm. Some biochemical extraction data suggest that no ARP is tightly associated (KraÈtzschmar
et al., 1996a), while other biochemical extraction data suggest
that a subset of ARP is tightly associated (Cohen et al., 2001) and
immuno¯uorescence detects ARP antigen on the heads and tails
of ejaculated human and monkey sperm (Hayashi et al., 1996;
Sivashanmugam et al., 1999). Human ARP is implicated in
human gamete membrane interactions by the observation that
human sperm treated with anti-human ARP antibodies show a
reduced ability to fertilize hamster oocytes (Cohen et al., 2001).
In addition, a recombinant form of this protein binds to the plasma
membrane of human oocytes (Cohen et al., 2001). However, it is
not known if there is a binding partner on the oocyte membrane
for sperm-associated CRISP proteins, and the mechanism by
which CRISP proteins participate in gamete membrane interactions remains uncharacterized.
Sperm ADAMs: fertilin b (ADAM2), fertilin a (ADAM1) and
cyritestin (ADAM3)
Fertilin b was ®rst implicated in gamete membrane interactions
because it was identi®ed as the antigen of an antibody, PH-30,
that blocked fertilization of guinea pig oocytes (Primakoff et al.,
1987). This antibody was made as described above, by
immunizing mice with a guinea pig sperm membrane preparation
to generate a battery of monoclonal antibodies that recognize
sperm surface antigens (Primakoff and Myles, 1983). Fertilin a
was identi®ed and characterized with fertilin b, since these two
proteins form a heterodimer (Primakoff et al., 1987; Waters and
White, 1997; Cho et al., 2000). In turn, cyritestin was identi®ed in
the mouse and monkey by various cloning strategies (Barker et
al., 1994; Wolfsberg et al., 1995; Heinlein et al., 1996). All have
since been identi®ed in rodents and primates, and fertilin b and
fertilin a have also been identi®ed in bovine and rabbit species.
The signi®cant homologies and conserved domain structure of
these and related proteins led to the identi®cation of the molecular
family known as ADAMs (an acronym for a disintegrin and a
metalloprotease domain); the terms MDC (for metalloproteasedisintegrin-cysteine rich), metalloprotease-disintegrin and cellular
disintegrin are also used in the literature. Fertilin a, fertilin b and
cyritestin are known as ADAM1, ADAM2 and ADAM3
respectively. Approximately 30 members of this molecular family
have been identi®ed to date in vertebrates, expressed in a wide
range of tissues and cell types. ADAM proteins are also present in
invertebrates such as Drosophila and Caenorhabditis elegans
(Primakoff and Myles, 2000). ADAM proteins, like the name
suggests, have a speci®c domain structure: a signal sequence,
prodomain, metalloprotease domain, disintegrin-like domain,
cysteine-rich domain, an epidermal growth factor (EGF)-like
repeat, and a transmembrane segment with a short cytoplasmic
tail. The disintegrin-like domains of these proteins have generated
great interest. These domains have homology to snake venom
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ligands for the integrin family of cell adhesion molecules (Gould
et al., 1990; McLane et al., 1998; Evans, 2001), suggestive of a
role for these sperm proteins in cell adhesion (discussed below).
Many of the snake venom polypeptides contain an RGD
tripeptide, similar to that recognized by eight of the 24 known
integrins (Evans, 2001; summary), although virtually no ADAMs
have an RGD in this region. Nevertheless, this region in
disintegrin domains is sometimes referred to as the `disintegrin
loop', since the RGD motif is on a loop structure in some snake
disintegrins.
The ADAM protein family has dual functionality. Some family
members have important proteolytic activities and others function
as cell adhesion molecules (Black and White, 1998; Primakoff
and Myles, 2000), although the speci®c functions of many of
these ADAM proteins remain to be fully characterized. This
review will focus exclusively on ADAM proteins involved in
adhesion between sperm and oocyte during fertilization, since
much has been learned about how fertilin b, fertilin a and
cyritestin participate in this cell±cell interaction. However, some
data suggest the possibility that metalloprotease activity (perhaps
of an ADAM or another protein) may play a role in gamete
membrane interactions (DõÂaz-PeÂrez et al., 1988; Correa et al.,
2000) and there are ADAM family members that are candidates
for this (Zhu et al., 1999, 2001).
Fertilin a, fertilin b and cyritestin undergo proteolytic
processing between the metalloprotease and disintegrin domains,
so that only the disintegrin domain, the cysteine-rich domain
and the EGF-like repeat remain on the surface of the mature
sperm (Figure 1). Fertilin a is processed intracellularly during
spermatogenesis in the testis (Lum and Blobel, 1997). Cyritestin
and fertilin b are cleaved during epididymal transit of the sperm
(Blobel et al., 1990; Linder et al., 1995; Hunnicutt et al., 1997;
Lum and Blobel, 1997; Yuan et al., 1997; Frayne et al., 1998).
These sperm ADAM proteins are localized to regions of the
sperm head (Phelps et al., 1990; Hardy et al., 1997; McLaughlin
et al., 1997; Yuan et al., 1997).
Numerous functional studies have provided evidence that
fertilin a, fertilin b and cyritestin participate in sperm±oocyte
adhesion. As noted above, an anti-fertilin b monoclonal antibody,
PH-30, blocked sperm±oocyte fusion of guinea pig oocytes
(Primakoff et al., 1987). Since that original study in 1987,
multiple types of reagents, including antibodies, peptides and
isolated proteins, have been used to examine the roles of these
proteins in gamete membrane interactions. Antibodies that crossreact with these proteins bind to sperm and inhibit fertilization in
IVF assays (Primakoff et al., 1987; Hardy et al., 1997; Yuan et
al., 1997). Recombinant forms of fertilin a, fertilin b and
cyritestin bind to the mouse oocyte plasma membrane, and in IVF
assays these proteins inhibit sperm±oocyte binding and reduce the
incidence of fertilization (Evans et al., 1997a,b, 1998; Bigler et
al., 2000; Zhu et al., 2000; Takahashi et al., 2001; Wong et al.,
2001; Eto et al., 2002; Zhu and Evans, 2002). In a similar fashion,
peptides corresponding to the fertilin b disintegrin loop inhibit
sperm binding and reduce fertilization in the mouse (Almeida et
al., 1995; Evans et al., 1995b; Yuan et al., 1997; Gupta et al.,
2000; Zhu et al., 2000; McLaughlin et al., 2001), guinea pig
(Myles et al., 1994) and human and non-human primate sperm
with ZP-free hamster oocytes (Gichuhi et al., 1997) and with
homologous oocytes (Bronson et al., 1999b; Mwethera et al.,
300
1999). Fertilin b disintegrin loop peptides and antibodies against
the fertilin b disintegrin loop also inhibit the binding of
recombinant fertilin b to oocytes (Evans et al., 1997a; Bigler et
al., 2000), indicating that fertilin b uses its disintegrin loop to
bind to cognate binding partners on the oocyte surface. There are
also similar data supporting the role of the mouse cyritestin
disintegrin loop (Linder and Heinlein, 1997; Yuan et al., 1997;
McLaughlin et al., 2001; Takahashi et al., 2001).
Structure±function studies of the disintegrin loops of mouse
fertilin b and cyritestin have revealed similar functional motifs:
ECDV in fertilin b (Bigler et al., 2000; Zhu et al., 2000;
Takahashi et al., 2001) and QCD with some involvement of
¯anking sequences in cyritestin (Takahashi et al., 2001). Recent
experimental data suggest that fertilin b and cyritestin could
recognize the same receptor (discussed in more detail below) (Eto
et al., 2002). Structure±function analysis of mouse fertilin a has
also been performed. Unlike fertilin b, a recombinant form of
fertilin a that lacks the disintegrin domain or has a truncated
disintegrin domain inhibits sperm±oocyte binding (Evans et al.,
1997b, 1998; Wong et al., 2001). This suggests that the cysteinerich domain and/or the EGF-like repeat of fertilin a participate in
fertilin a-mediated cell adhesion; this is similar to what has been
observed for ADAM12 (Iba et al., 1999). A recombinant form of
the fertilin a disintegrin domain also binds to mouse oocytes and
inhibits sperm±oocyte binding (Wong et al., 2001), demonstrating
that this domain is also involved in fertilin a-mediated adhesion.
Interestingly, peptides from the fertilin a disintegrin loop do not
inhibit sperm±oocyte binding (Yuan et al., 1997) or the binding of
recombinant fertilin a disintegrin domain to oocyte (Wong et al.,
2001), raising the possibility that other amino acids are involved
in the interaction of the fertilin a disintegrin domain with cognate
binding partners on the oocyte surface.
Important insights have come from studies of fertilin b and
cyritestin knockout mice. Sperm from both fertilin b and
cyritestin knockout mice show reduced binding to the oocyte
plasma membrane, although some of the few sperm that bind are
able to fuse with the oocyte membrane and fertilize and activate
oocytes (Cho et al., 1998; Nishimura et al., 2001). Surprisingly,
sperm from these knockout male mice have some other,
unanticipated defects. In addition to adhering poorly to the
oocyte plasma membrane, sperm from fertilin b and cyritestin
knockout mice also adhere poorly to the ZP (Cho et al., 1998;
Shamsadin et al., 1999; Nishimura et al., 2001), and fertilin b
knockout sperm show de®cient migration from the uterus to the
oviduct (Cho et al., 1998). Considering that fertilin b knockout
males mate normally and their sperm have normal motility, this
suggests a defect in sperm migration to the oviduct or interaction
with the oviduct walls. [Cyritestin knockout sperm, in contrast,
have normal transit into the oviduct (Shamsadin et al., 1999;
Nishimura et al., 2001).] Does this mean that fertilin b or
cyritestin is involved with ZP binding, or that fertilin b is
involved in migration to or interaction with the oviduct? This is
one possible interpretation, but there are others. Analysis of
protein expression pro®les of fertilin b and cyritestin knockout
sperm reveals that: (i) fertilin b knockout sperm lack fertilin a
and have greatly reduced amounts of cyritestin; and (ii) cyritestin
knockout sperm lack fertilin a and have ~50% of the wild-type
amount of fertilin b (Nishimura et al., 2001). These multiple
molecular de®ciencies are likely to contribute to the defects in
Gamete plasma membrane interactions
Figure 1. Schematic diagram of sperm and oocyte molecules known to participate in gamete membrane interactions. The diagram shows the approximate
relationship of sperm and oocyte adhesion molecules and associated membrane proteins. As noted in the text, CRISP1 is not an integral membrane protein of the
sperm, but a protein that associates with the sperm (by an as yet uncharacterized mechanism) during epididymal maturation. It is not known if there is a binding
partner for CRISP1 on the sperm or on the oocyte (it is drawn as a peripheral membrane protein on the sperm, rather than receptor-bound). Fertilin a and fertilin b
are shown associated as a dimer (Primakoff et al., 1987; Waters and White, 1997; Cho et al., 2000). Asterisks next to domains of fertilin a, fertilin b and cyritestin
indicate the domains that participate in adhesion mediated by these molecules (Evans et al., 1998; Bigler et al., 2000; Zhu et al., 2000; Takahashi et al., 2001;
Wong et al., 2001). A single integrin is shown in the oocyte plasma membrane as an a/b heterodimer. This integrin could be one of several integrins (a9b1 and
possibly avb1, avb3, a6b1; see text for details). The ligand binding site of integrins is composed of regions in the globular heads of both the a and b subunits
(additional information in Evans, 2001); the heads are presented on extended stalks (Xiong et al., 2001). EGF = epidermal growth factor.
gamete membrane interactions and possibly also to other sperm
function de®ciencies. It can also be speculated that the ZP binding
defect and/or oviductal transit are disrupted due to protein
expression abnormalities. In other words, sperm from fertilin b
knockout males that lack fertilin b, fertilin a and cyritestin could
also lack a protein critical for interaction with the ZP or the
oviduct.
Why might sperm from these knockout mice have abnormal
protein expression pro®les? Some insight comes from a different
knockout mouse, lacking a chaperone protein (called calmegin)
expressed in the endoplasmic reticulum of developing sperm.
Sperm from calmegin knockout males have almost exactly the
defects in sperm function as do sperm from fertilin b knockout
males: reduced transit into the oviduct, reduced adhesion to the
ZP, and reduced adhesion to the oocyte plasma membrane (Ikawa
et al., 1997, 2001). Sperm from calmegin knockout males also
have greatly reduced levels of fertilin b (Ikawa et al., 2001),
indicating that this knockout also has an abnormal protein
expression pro®le. Calmegin appears to function as a chaperone to
help protein folding; it interacts with newly synthesized proteins
(including fertilin a and fertilin b) in the endoplasmic reticulum
during spermatogenesis (Ikawa et al., 1997, 2001). This suggests
that in a calmegin knockout mouse, fertilin a and fertilin b are left
to `fend for themselves' in the endoplasmic reticulum, and this in
turn leads to abnormalities, including the disappearance of fertilin
b from sperm (Ikawa et al., 2001). It can be speculated that other
proteins are also adversely affected, by misfolding and/or by
misexpression. These problems could then lead to the fertilization
function defects observed in calmegin knockout male mice. The
angiotensin-converting enzyme (ACE) knockout also has a
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similar phenotype. ACE is present in the blood, is expressed in the
testis, and is also present in epididymal ¯uid. Sperm from ACE
knockout mice have defects in transit up the oviduct and in ZP
binding (Hagaman et al., 1998). Whilst it is unclear exactly why
the ACE knockouts have these defects, one possibility is that
sperm from these mice might have an abnormal protein pro®le on
their surfaces due to defects in epididymal maturation.
To date, functional fertilin a and cyritestin genes have not been
identi®ed in humans. Fertilin a and cyritestin genes were
determined to be non-functional pseudogenes (Jury et al., 1997,
1998; Frayne and Hall, 1998; Grzmil et al., 2001), having
numerous point mutations that introduce frameshifts and
premature stop codons and thus being unable to produce
functional protein. In fact, several human testis-speci®c ADAM
genes are intronless or non-functional pseudogenes (Cerretti et
al., 1999; Frayne et al., 1999; Poindexter et al., 1999). The fact
that humans are essentially fertilin a and cyritestin knockouts
(lacking functional fertilin a and cyritestin genes) and yet have a
different phenotype from cyritestin knockout mice suggests at
least two possibilities. It could be that fertilin a and cyritestin are
not essential for human gamete membrane interactions, and are
also not required for a proper sperm protein expression pro®le.
While fertilin a forms heterodimers with fertilin b in guinea pig,
bovine and mouse sperm (Primakoff et al., 1987; Blobel et al.,
1990; Waters and White, 1997; Cho et al., 2000), it could be that
fertilin b functions by itself in human sperm, as a monomer (or
homomultimer) without fertilin a. Alternatively, it could be that
some other proteinÐperhaps another ADAM proteinÐplays the
same role and substitutes for fertilin a and cyritestin in human
sperm. Thirteen members of the ADAM family are expressed
predominantly in the testis [fertilin b, cyritestin, ADAM5
(tMDCII), ADAM6, ADAM16 (xMDC16), ADAM18
(tMDCIII), ADAM20, ADAM21, ADAM24 (testase 1),
ADAM25 (testase 2), ADAM26 (testase 3), ADAM29,
ADAM30]. Five of these testis ADAMs (fertilin b, cyritestin,
ADAM5, ADAM16, ADAM18) are known to be expressed as
proteins on male germ cells and/or mature sperm in at least one
species. Several ADAMs, including some that are expressed in the
testis, are similar to and closely related to fertilin a or cyritestin
(Poindexter et al., 1999; Yoshinaka et al., 2002; for phylogenetic
trees of ADAM family members), and two fertilin a genes,
dubbed ADAM1a and ADAM1b, were recently identi®ed in the
mouse (Nishimura et al., 2002). An additional candidate is
ADAM15, as recombinant mouse ADAM15 disintegrin domain
was recently shown to bind to oocytes and inhibit sperm±oocyte
binding (Eto et al., 2002). It should be noted that this effect could
be non-speci®c, since ADAM15, fertilin b, cyritestin, fertilin a
and other ADAMs have been proposed to recognize the same
receptor, the integrin a9b1 (discussed in more detail below). It is
not known if ADAM15 protein is expressed on sperm; ADAM15
mRNA is detected in Northern blots of human and mouse testis
and many other tissues (KraÈtzschmar et al., 1996b; Lum et al.,
1998). ADAM5 is not a candidate to participate in human gamete
membrane adhesion, as it is also a pseudogene in humans (Frayne
et al., 1999).
Oocyte integrins
Integrins as well as other cell adhesion molecules (e.g. cadherins,
immunoglobulin superfamily members, etc.) have been specu-
302
lated to mediate sperm±oocyte adhesion, as they mediate somatic
cell adhesion. In particular, ever since the identi®cation of an
integrin ligand-like, disintegrin domain in fertilin b and cyritestin
[and later in fertilin a (Lum and Blobel, 1997; Waters and White,
1997)], it was hypothesized that that these sperm ligands could
bind to integrins on the oocyte membrane. Therefore, the issue of
the role of oocyte integrins in gamete membrane interactions has
two aspects: (i) whether oocyte integrins participate in gamete
interactions in general; and (ii) whether oocyte integrins
recognize sperm ADAMs.
Integrins are a family of heterodimeric cell adhesion molecules
that mediate cell±cell and cell±extracellular matrix interactions.
To date, 18 a subunits and eight b subunits have been identi®ed,
and these combine to make 24 different integrins. These 24
integrins can be divided into six subfamilies based on sequence
homologies between the a subunits and the general characteristics
of ligands recognized by integrin heterodimers containing each a
subunit.
Several integrin subunits have been detected in mammalian
oocytes at the mRNA or protein level, including a2, a3, a4, a5,
a6, av, a9, aM, b1, b2, b3 and b5 (Fusi et al., 1992, 1993;
Anderson et al., 1993; Tarone et al., 1993; Almeida et al., 1995;
Evans et al., 1995a; de Nadai et al., 1996; Linfor and Berger,
2000; Neilson et al., 2000; Stanton and Green, 2001). The
involvement of the RGD-binding subfamily of integrins (a5b1,
a8b1, aVb1, aVb3, aVb5, aVb6, aVb8, aIIbb3) has been suggested
by the inhibition of interactions of human and hamster sperm with
ZP-free hamster and human oocytes by RGD peptides (Bronson
and Fusi, 1990; Ji et al., 1998). RGD peptides do not have a
substantial inhibitory effect on mouse gamete interactions
(Almeida et al., 1995; Evans et al., 1995b), although they do
have a modest inhibitory effect on the binding of recombinant
fertilin b to mouse oocytes (Zhu and Evans, 2002). Biochemical
analyses have implicated av and b1 integrin subunits on the pig
oocyte in the recognition of isolated pig sperm membrane proteins
(Linfor and Berger, 2000). A few anti-integrin antibodies have
been reported to affect gamete interactions in IVF assays. Anti-b1
antibodies have a moderate inhibitory effect on mouse, human
and pig sperm±oocyte interactions (Evans et al., 1997a; Ji et al.,
1998; Linfor and Berger, 2000) and also on the binding of
recombinant fertilin b to mouse oocytes (Evans et al., 1997a). An
anti-a6 function-blocking monoclonal antibody, GoH3, inhibits
mouse sperm±oocyte interactions in some (Almeida et al., 1995)
but not all IVF assays (Evans et al., 1997a; Evans, 1999; Miller et
al., 2000). Moreover, the ®ndings that oocytes from a6 knockout
mice and oocytes with undetectable amounts of a6 GoH3 epitope
are capable of being fertilized strongly suggest that oocyte a6 is
not required for fertilization (Evans et al., 1997a; Miller et al.,
2000). It should be noted that it is extremely dif®cult to study
oocytes from integrin knockout mice, since many integrins are
widely expressed in the body and thus many integrin knockouts
die pre- or peri-natally, well before reaching sexual maturity
(Hynes, 1996; De Arcangelis and Georges-Labouesse, 2000). As
a6 knockout mice die shortly after birth, the study of oocytes from
the a6 knockout mice required the isolation of ovaries from
newborns, and transplanting these ovaries into the kidney capsule
bursal cavity of ovariectomized mice to allow the ovaries to
develop to the point where oocytes could be retrieved from them
(Cox et al., 1996; Miller et al., 2000).
Gamete plasma membrane interactions
Despite these technical challenges, there are other ways to
examine the possible role of oocyte integrins in gamete membrane
interactions. One way is to consider data from other experimental
systems. To date, eight different ADAM family members have
been reported to interact with six different integrins (Table I),
based primarily on assays of adhesion of integrin-expressing cells
to ADAM proteins, in some cases combined with antibody and
peptide inhibition studies. Members of three different integrin
subfamilies appear to interact with ADAMs, with several of these
integrins expressed by oocytes. With these ®ndings in mind, what
are the data that suggest these oocyte integrins could actually
interact with an ADAM on sperm?
a4b1 and a9b1 are members of the same integrin subfamily;
this subfamily has three members (a4b1 a4b7, a9b1). These
integrins recognize a diverse array of ligands, including vascular
cell adhesion molecule (VCAM)-1, osteopontin, tenascin-C,
®bronectin and Mad-CAM-1, and in several cases recognizing a
conserved amino acid sequence motif containing an Asp residue
in these proteins (Evans, 2001; summary). As shown in Table I,
seven of the eight ADAMs thus far demonstrated to bind to an
integrin, including the three sperm ADAMs (fertilin a, fertilin b,
cyritestin), can interact with a member of the a4/a9 subfamily
(Eto et al., 2000, 2002; Bridges et al., 2002). With regard to
gamete membrane interactions, a member of the a4/a9 subfamily
has been implicated by two results. Somatic cells expressing a9b1
adhere to recombinant fertilin a, fertilin b and cyritestin (Eto et
al., 2002). In addition, the binding of recombinant fertilin b to
mouse oocytes is inhibited by a peptide sequence, MLDG, that
perturbs adhesion mediated by the a4/a9 integrins (Zhu and
Evans, 2002). The a4 subunit has been reported to be present on
hamster oocytes (de Nadai et al., 1996), but expression is weak or
Table I. ADAM±integrin interactions
Integrin Integrin subfamily (members)
a9b1
a6b1
avb3
avb5
a5b1
a4b1
ADAMs (reference)
a4/a9 subfamily (a4b1 a4b7, a9b1) Fertilin a (Eto et al., 2002)
Fertilin b (Eto et al., 2002)
Cyritestin (Eto et al., 2002)
ADAM9 (Eto et al., 2002)
ADAM12 (Eto et al., 2000)
ADAM15 (Eto et al., 2000,
2002)
Fertilin b (Chen and
Laminin binding (a3b1, a6b1,
Sampson, 1999;
a6b4, a7b1)
Bigler et al., 2000)
Cyritestin (Takahashi et al.,
2001)
ADAM9 (Nath et al., 2000)
RGD binding (a5b1, a8b1,
ADAM15 (Zhang et al., 1998;
aVb1, aVb3, aVb5, aVb6,
Nath et al., 1999)
aVb8, aIIbb3)
ADAM23 (Cal et al., 2000)
RGD binding (a5b1, a8b1,
ADAM9 (Zhou et al., 2001)
aVb1, aVb3, aVb5, aVb6, aVb8,
aIIbb3)
RGD binding (a5b1, a8b1,
ADAM15 (Nath et al., 1999)
aVb1, aVb3, aVb5, aVb6, aVb8,
aIIbb3)
ADAM28 (Bridges et al.,
a4/a9 subfamily (a4b1
a4b7, a9b1)
2002)
absent on human oocytes (Campbell et al., 1995; de Nadai et al.,
1996) and mouse oocytes (J.P.Evans, unpublished data). a9
cDNA is detected in a pool of expressed sequence tags from
mouse oocyte cDNA libraries (Stanton and Green, 2001),
although the protein has not been detected. Thus, the possibility
remains that an integrin related to but distinct from a4b1, a9b1
and a4b7 is expressed by oocytes and mediates adhesion to sperm
ADAMs. The identity of this oocyte integrin remains to be
de®nitely determined.
a6b1 is a member of the subfamily of integrins that primarily
recognize laminins, and has been suggested to participate in
fertilization (Almeida et al., 1995; Bigler et al., 2000; Takahashi
et al., 2001). It is also implicated as a receptor for fertilin b and
cyritestin based on a chemical cross-linking study with a fertilin b
peptide (Chen and Sampson, 1999) and from studies with an antia6 function-blocking monoclonal antibody (Almeida et al., 1995;
Bigler et al., 2000; Takahashi et al., 2001). However, other
attempts to inhibit interactions of fertilin b and other ADAMs
with oocytes and other cells using the same anti-a6 antibody have
been unsuccessful (Evans et al., 1997a; Eto et al., 2002; Zhu and
Evans, 2002). It is a formal possibility that fertilin b recognizes a
site on a6b1 that is not blocked by this anti-a6 antibody. However,
as noted above, a6 is not required for fertilization (Miller et al.,
2000). More recently, there has been speculation that oocyte a6b1
has a role in ADAM binding perhaps not as a direct binding
partner but as a component of a multi-molecular complex in the
oocyte plasma membrane (Bigler et al., 2000; Takahashi et al.,
2001); this is addressed in more detail in the section below on
oocyte tetraspanins.
Finally, aVb3, avb5 and a5b1 are members of the RGD-binding
subfamily that mediate adhesion to ligands such as ®bronectin,
vitronectin and ®brinogen by binding to an RGD motif in the
ligand in most cases. Members of the RGD-binding integrin
subfamily are expressed by oocytes (see above) and are
implicated in fertilization by studies using RGD peptides in IVF
assays (Bronson and Fusi, 1990; Ji et al., 1998) and other work
(Linfor and Berger, 2000). With regard to the potential role of
these integrins in the recognition of a sperm ADAM, RGD
peptides have a moderate inhibitory effect on the binding of
fertilin b to mouse oocytes (Zhu and Evans, 2002). While one
possible explanation for this is that an RGD-binding integrin
contributes to fertilin b binding, it is also possible that the RGD
peptide is weakly perturbing an a4/a9 integrin on the oocyte
surface (as these integrins recognize sequences in some ligands
that have some weak similarities to RGD motifs). Therefore, the
exact roles of RGD-binding integrins in gamete sperm ADAMmediated adhesion as well as the identi®cation of speci®c family
members on oocytes that could be involved remain to be fully
characterized.
Oocyte tetraspanins
Tetraspanins (also known as tetraspans or TM4SF proteins) are a
family of proteins with the conserved structural features of four
membrane-spanning regions (hence the name) and two extracellular loops (one large, one small), with a series of other
conserved residues (Berditchevski, 2001; Boucheix and
Rubinstein, 2001; Hemler, 2001). It should be noted that not
every protein with four membrane-spanning regions is a bona ®de
tetraspanin. To date, 28 mammalian tetraspanin family members
303
J.P.Evans
have been described, and several of these are implicated in a
variety of cellular and physiological processes, such as cell
adhesion, motility, proliferation and differentiation (Boucheix and
Rubinstein, 2001; Hemler, 2001). Tetraspanins in general do not
appear to function as receptors for extracellular ligands, but they
do associate in the plane of the lipid bilayer with other membrane
proteins, including other tetraspanins, integrins, immunoglobulin
superfamily members, proteoglycans, complement regulatory
proteins, growth factors receptors and others (Hemler, 1998;
Berditchevski and Odintsova, 1999; Woods and Couchman, 2000;
Boucheix and Rubinstein, 2001; Hemler, 2001). These intramembrane (or lateral or cis) associations form complexes referred to as
tetraspanin webs.
One member of this family, CD9, plays a key role in gamete
membrane interactions. Although CD9 is widely expressed in the
body, the phenotype of the knockout mouse shows speci®c effects
on female fertility. CD9 knockout females appear to ovulate
normally, and yields of oocytes from super-ovulated animals are
similar to yields from knockouts. Oocyte maturation to metaphase
II also appears to be normal. However, oocytes from CD9
knockout mice are rarely fertilized (Kaji et al., 2000; Le Naour et
al., 2000; Miyado et al., 2000). Sperm are observed in the
perivitelline space of ZP-intact oocytes fertilized in vivo or in
vitro, apparently unable to undergo sperm±oocyte fusion and lead
to fertilization and oocyte activation (Le Naour et al., 2000;
Miyado et al., 2000). Sperm are able to adhere to the plasma
membrane of ZP-free CD9 knockout oocytes, but very rarely fuse
with the oocyte membrane [between three reports, only three of
246 ZP-free oocytes were fertilized (Kaji et al., 2000; Le Naour et
al., 2000; Miyado et al., 2000)]. However, CD9 knockout females
did, on rare occasions, conceive and give birth to pups. In
addition, oocytes from CD9 knockout mice could be fertilized by
ICSI and these embryos developed to term (Miyado et al., 2000).
If the expression of CD9 is induced in CD9 knockout oocytes (via
microinjection of CD9 mRNA), the ability of the oocytes to be
fertilized can be restored (Zhu et al., 2002). Using this method, a
structure±function analysis of CD9 has been performed. Two
mutated forms of CD9, a point mutation of amino acid 174 and
the triple mutation of amino acids 173±175 [F174A and
SFQ(173±175)AAA respectively] very poorly rescue the ability
of CD9 knockout oocytes to be fertilized (Zhu et al., 2002),
suggesting that the SFQ motif in this region has a critical function
in mouse oocyte CD9.
The observation that sperm adhere to the CD9 knockout oocyte
plasma membrane but do not fuse is suggestive of a role for CD9
in sperm±oocyte fusion. Questions have been raised as to whether
CD9 may play a role, direct or indirect, in sperm±oocyte
adhesion. IVF results with anti-CD9 antibodies are con¯icting;
ZP-free oocytes treated with anti-CD9 monoclonal antibodies
have been reported to have reduced numbers of bound sperm
(Chen et al., 1999; Takahashi et al., 2001), whereas another report
states that sperm binding was unaffected by an anti-CD9 antibody
(Miller et al., 2000). Additionally, treatment of oocytes with a
recombinant form of the large extracellular loop of CD9 leads to a
reduction in sperm±oocyte fusion when the oocytes are
inseminated, with no effects on sperm±oocyte binding (Zhu et
al., 2002). Other data do suggest a role for CD9 in sperm±oocyte
adhesion. ZP-free oocytes treated with anti-CD9 antibodies show
reduced levels of binding of the sperm ligands cyritestin
304
(Takahashi et al., 2001), fertilin a (Wong et al., 2001) and
fertilin b (Chen et al., 1999; Zhu and Evans, 2002). The inhibitory
activity of anti-CD9 antibodies on the binding of fertilin a and
fertilin b depends on the presentation of the sperm ligand to
oocytes in the binding assays. The binding of multimeric forms
(immobilized on small beads) of fertilin a and fertilin b is
inhibited by an anti-CD9 antibody, whereas soluble monomeric
forms of these sperm ligands still appear to bind (Wong et al.,
2001; Zhu and Evans, 2002). Similar inhibition of binding of
beads coated with cyritestin and fertilin b has been observed with
a different anti-CD9 antibody (Chen et al., 1999; Takahashi et al.,
2001). While the signi®cance of this is unclear, it could be
suggestive of a role for CD9 to strengthen adhesions mediated by
these sperm ADAMs rather than the initial molecular interaction
of sperm ligand to oocyte receptor. The connection between this
and the ®nding that sperm can bind to CD9 knockout oocytes is
not known, although it is likely that the action of functionblocking antibodies is distinctly different from the effect of
having no CD9 present in the oocyte membrane. Furthermore,
results with different anti-CD9 monoclonal antibodies (Chen et
al., 1999; Miller et al., 2000; Takahashi et al., 2001; Wong et al.,
2001; Zhu and Evans, 2002) as well as structure±function analysis
of CD9 (Zhu et al., 2002) raise the possibility that different
portions of CD9 may have functions in adhesion and fusion.
Tetraspanins, as noted above, are components of multimolecular complexes or networks in the plasma membrane.
These networks include integrins, other types of tetraspanins, and
other membrane proteins. Several different integrins are expressed by oocytes (see details above), and at least seven different
tetraspanins (CD9, CD81, CD82, CD151, SAS, Tspan-3 and
Tspan-5) and various tetraspanin-associated proteins are potentially expressed in mouse or human oocytes (Neilson et al., 2000;
Stanton and Green, 2001). The participation of some of these
network components in fertilization is beginning to be examined.
CD81 is expressed on the mouse oocyte membrane, but antiCD81 antibodies do not inhibit sperm or recombinant cyritestin
binding to oocytes, whereas anti-CD9 antibodies do inhibit these
interactions (Takahashi et al., 2001). However, there is some
suggestion that CD81 knockout mice may have fertility problems
[reduced reproductive capacity after repeated back-crosses (Deng
et al., 2000)], although the cause of this is not known. Antibodies
that cross-react with CD98, another type of protein that associates
with integrins and tetraspanins, inhibit sperm or recombinant
cyritestin binding to oocytes (Takahashi et al., 2001). Finally, as
discussed above, the integrin a6b1 has been implicated in oocyte
interactions with fertilin b and cyritestin on sperm (Bigler et al.,
2000; Takahashi et al., 2001), although studies of oocytes from a6
knockout mice demonstrate that a6 expression by oocytes is not
required for fertilization (Miller et al., 2000). In other cell types,
a6b1 can interact with several members of the tetraspanin family
(CD9, CD63, CD81, CD82, CD151) and many tetraspanins can
interact laterally with at least one member of the laminin-binding
family of integrins [a6b1, a3b1, a6b7 and a7b1 (Berditchevski,
2001; Boucheix and Rubinstein, 2001)]. CD9 and a6b1 can
associate in the mouse oocyte membrane (Miyado et al., 2000),
although oocytes lacking a6 still express CD9 on their plasma
membranes and fertilization of these oocytes is inhibited with
anti-CD9 antibodies (Miller et al., 2000). Relatively little is
known about tetraspanin complexes in the oocyte membrane, but
Gamete plasma membrane interactions
further characterization of these complexes in oocyte membranes
should shed light on the mechanisms of action of these molecules
in mediating adhesion and fusion with sperm.
Other candidate molecules
Fertilin a and fertilin b are two of the best characterized antigens
identi®ed by screening a battery of sperm surface monoclonal
antibodies. However, it is worth noting that although many more
gamete antigens have been identi®ed by this method, these
antigens and/or their role in fertilization has not been characterized as extensively as those of the PH-30 antigen. Examples
include the sperm antigens for monoclonal antibody MH61
(Okabe et al., 1990) [which has been identi®ed as CD46/
membrane cofactor protein (MCP) (Okabe et al., 1992)],
monoclonal antibody MN9 [the antigen of which has been named
equatorin (Toshimori et al., 1992, 1998)] and others (Saling et al.,
1985; Okabe et al., 1988; Allen and Green, 1995; Noor and
Moore, 1999). CD46 is one protein that can laterally associate
with integrins and tetraspanins (Lozahic et al., 2000), although
CD46 was not detected in the membranes of human oocytes
(Fenichel et al., 1995). There are other candidate molecules
hypothesized to be involved in gamete membrane interactions.
These include extracellular matrix molecules, sulphoglycolipids
and components of the complement pathway, including C3b and
its receptors CD46/MCP and the integrin aMb2, and also C1q and
its receptors (Evans, 1999; review). A GPI-anchored protein(s) on
oocytes has been implicated by a study in which oocytes treated
with phosphatidylinositol-speci®c phospholipase C (which removes GPI-anchored proteins) show a greatly reduced ability to
support sperm adhesion and fusion (Coonrod et al., 1999). A 94
kDa protein on oocytes has been implicated by studies in which
oocytes treated with proteases show a greatly reduced ability to
support sperm adhesion and fusion, and reappearance of a 94 kDa
protein correlated with recovery of the ability of the oocytes to be
fertilized (Kellom et al., 1992). A metalloprotease (on either
sperm or oocyte) has been implicated by studies in which
treatment of gametes with metalloprotease inhibitors at the start of
insemination leads to decreased fertilization (Correa et al., 2000).
A model of how mammalian sperm±oocyte adhesion and
fusion may occur
Gamete membrane interactions involve cell adhesion and then
membrane fusion. Fertilin a, fertilin b and cyritestin on sperm
and integrins on oocytes appear to mediate the adhesion process.
CRISP1 may be involved in adhesion or fusion. CRISP1 protein
puri®ed from rat epididymal extracts binds to the oocyte plasma
membrane and inhibits sperm±oocyte fusion in IVF of ZP-free rat
oocytes, apparently without affecting sperm±oocyte adhesion
(Rochwerger et al., 1992). However, CRISP1 does not have
homology to proteins known to be involved in membrane fusion
events, and current speculation places CRISP1 upstream of fusion
(Cuasnicu et al., 2001). Fertilin a was originally a candidate to
mediate membrane fusion between the gametes (Blobel et al.,
1992), but the ®nding that the fertilin a-lacking sperm from
fertilin b and cyritestin knockout mice are still capable of
membrane fusion has dispelled this theory (Shamsadin et al.,
1999; Nishimura et al., 2001). CD9 has been implicated in certain
types of membrane fusions (Schmid et al., 2000; Boucheix and
Rubinstein, 2001), but it is not known if CD9 in the oocyte has a
direct role in facilitating gamete fusion, or has an upstream action
such as by enhancing sperm interactions or modulating the oocyte
membrane environment to make it `fusion competent'. Results
with two different monoclonal antibodies (Chen et al., 1999;
Miller et al., 2000; Takahashi et al., 2001; Wong et al., 2001; Zhu
and Evans, 2002) as well as structure±function analysis of CD9
(Zhu et al., 2002) raise the possibility that different portions of
CD9 may have functions in sperm adhesion or gamete fusion,
although this remains to be fully elucidated. Nevertheless, one
intriguing theme that has emerged is the importance of multimeric
complexes and a proper membrane environment on both sperm
and oocyte for gamete membrane interactions to be successful. In
the case of the oocyte, CD9 appears to be a key component of
these multimeric complexes. In the case of sperm, insight into the
importance of multimeric membrane protein complexes has come
from analyses of protein expression pro®les of sperm from the
fertilin b and cyritestin knockout mice (Nishimura et al., 2001).
Disruption of the membrane environment by an antibody
treatment, a gene knockout removing a key component or other
means, appears to lead to suboptimal membrane order that is less
capable of supporting gamete adhesion and/or fusion.
The adhesion of sperm to oocyte is frequently cited as likely to
be analogous to the mechanism by which leukocytes interact with
endothelial cells. In this cell±cell interaction system, the
adhesions occur in a step-wise fashion, starting with initial
attachments (also called rolling or tethering) and leading to ®rm
adhesion bringing the two membranes into close apposition;
®nally, the leukocytes undergo extravasation, traversing the
endothelium to exit from the bloodstream. In the leukocyte±
endothelium system, distinct ligand±receptor pairs with speci®c
kinetics and af®nities mediate each of the speci®c adhesion steps.
Rolling is mediated by selectin±carbohydrate interactions that
have rapid kinetics, followed by ®rmer adhesions supported ®rst
by the integrin a4b1 on the leukocyte binding endothelial VCAM1, and then interactions of aLb2 and aMb2 with intracellular
adhesion molecules (Brown, 1997; Worthylake and Burridge,
2001). Sperm±oocyte membrane interactions also appear to occur
in a step-wise fashion, in terms of the spatial domains of the
sperm head that interact with the oocyte (see above) and possibly
also in terms of the molecules involved.
A hypothetical model for how gamete membrane adhesion
leads to fusion is shown in Figure 2. This model is based on
membrane fusion events between cells and virus particles (Lentz
et al., 2000; Eckert and Kim, 2001). In many virus±cell
interactions, membrane fusion is mediated by a viral fusion
protein in the membrane of the viral envelope. The viral fusion
protein contains a hydrophobic subdomain, called a fusion
peptide, which is folded within the fusion protein so that its
hydrophobic amino acids are not exposed to the aqueous
environment until a conformation change occurs to expose it. In
the hypothetical model in Figure 2, panel A shows adhesion
mediated by receptor±ligand pairs labelled 1 and 2; the adhesion
between receptor±ligand pair 2 has induced bending of the lipid
bilayers, bringing the two membranes into closer apposition. A
putative fusion protein is shown with the hydrophobic fusion
peptide concealed. (Please note that the two plasma membranes
shown are purposely not identi®ed as sperm and oocyte, since the
mechanism could work with these proteins in either gamete.) In
305
J.P.Evans
bilayers intermingle, a state called hemifusion (shown in panel C).
Finally, panel D shows the formation of an opening, called a
fusion pore, between the two membranes, connecting the
cytoplasms of the two cells. The fusion pore then expands,
ultimately incorporating one membrane into the other. It should
be emphasized that this is only a possibility for how mammalian
gamete membrane interactions occur, although recent analysis of
invertebrate sperm proteins has identi®ed hydrophobic regions
that might function by a mechanism somewhat similar to that of
viral fusion peptides (Ulrich et al., 1998; Glaser et al., 1999;
Kresge et al., 2001).
A second model for membrane fusion comes from studies of
membrane fusion between intracellular vesicles with other
membranes during traf®cking, mediated by transmembrane
proteins known as soluble N-ethylmaleimide-sensitive factor
(NSF) attachment protein receptor (SNARE) proteins and
cytoplasmic accessory proteins that regulate SNARE function
(Lentz et al., 2000; Chen and Scheller, 2001). A v-SNARE on a
vesicle interacts with a t-SNARE on a target membrane, and these
are believed to intertwine and form bundles of a-helices, bringing
the vesicle and target membranes into close apposition, analogous
to the way in which a viral fusion protein brings membranes close
together after insertion of the fusion peptide, leading to formation
of the fusion pore. It is possible that there are SNARE-like
proteins on the extracellular surface of the sperm and oocyte that
mediate gamete fusion. However, it should be emphasized that the
SNARE model is based on membrane fusion events that occur on
the interior of cells, with fusion being initiated by the membrane
proteins and lipids that face the cytoplasm. Sperm±oocyte (and
other cell±cell) fusion events are different in this regard,
occurring on the extracellular surfaces of membranes and thus
initiated by membrane proteins and lipids that face the
extracellular space. While some SNARE machinery has been
detected in gametes and is postulated to mediate acrosome and
cortical granule exocytosis, it is unclear if it participates in plasma
membrane fusion (Ramalho-Santos et al., 2000). Finally,
metalloprotease activity may play a role in gamete membrane
fusion (DõÂaz-PeÂrez et al., 1988; Correa et al., 2000)
Concluding thoughts
Figure 2. A hypothetical model for how gamete membrane adhesion and
fusion occur. (A) Adhesion mediated by receptor±ligand pairs labelled 1 and
2; the adhesion between receptor±ligand pair #2 has induced bending of the
lipid bilayers. A putative fusion protein is shown with the hydrophobic fusion
peptide concealed (labelled 3). (B) The fusion protein undergoes a
conformational change to expose the fusion peptide, which then inserts in
the opposing bilayer. (C) Hemifusion, the fusion of the outer lea¯ets of the
two membranes. (D) The formation of a fusion pore (fusion of outer and inner
lea¯ets) between the two membranes. See text for more details.
panel B, the fusion protein has undergone a conformational
change to expose the fusion peptide, which then inserts in the
opposing bilayer. Next, the facing outer lea¯ets of two lipid
306
The advancement of our knowledge of the molecular basis of
events surrounding fertilization, including gamete membrane
interactions, has paled in comparison with the speed with which
assisted reproductive technologies have been developed. The
most obvious procedure that affects gamete membrane interactions is ICSI, which actually bypasses membrane interactions
altogether. Other clinical procedures may impact fertilization as
well, such as by affecting oocyte or sperm maturation. For
example, expression of CRISP1 mRNA and protein in the rat
epididymis is reduced after androgen ablation (Cameo and
Blaquier, 1976; Roberts et al., 2001), after depletion of testicular
factors from epididymal ¯uid (Turner and Bomgardner, 2002) and
after vasectomy (Turner et al., 1999), and is not recovered after
vasovasostomy (Turner et al., 2000). This may have implications
for male infertility and for patients hoping to recover fertility after
vasovasostomy. It is not known (but is conceivable) that other
epididymal functions, such as processing of fertilin b or cyritestin,
Gamete plasma membrane interactions
might also be adversely affected by vasectomy and not recovered
after vasovasostomy.
Proper gamete membrane interactions may do much more than
simply merge two cells into one. These cellular processes
introduce the paternal DNA, the sperm centriole and perhaps an
oocyte activating factor to the oocyte in a precisely regulated
fashion. While ICSI does result in oocyte activation, plasma
membrane interactions between gametes may be important for
correct temporal and spatial patterns of calcium signalling upon
oocyte activation (Tesarik et al., 1994; Nakano et al., 1997; Sato
et al., 1999; Deguchi et al., 2000) and/or for patterning of early
embryonic development (Piotrowska and Zernicka-Goetz, 2001).
Thus, plasma membrane interactions between the gametes as well
as other events around the time of conception could contribute to
the success (or failure) of embryo and fetal development.
Acknowledgements
I am indebted to members of my research group for their insights, enthusiasm
and dedication. These studies and research have been supported by the
National Institutes of Health (HD 37696 and training grant funds from HD
07276 and CA 09110).
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